The syms x is not having any effect there. What is happening is that you are encountering an undocumented syntax that creates an inline() object for some kinds of character vectors.
The syms x is having an effect there, so eqn is created as a symbolic expression.
That undocumented syntax... when you happen to pass in a character vector that looks like an identifier, the internal code turns it into a function handle; for example fzero('cos', 2) would turn into fzero(@cos,2) effectively. But there is no visible function named eqn so that will fail.
fzero is not defined for symbolic expressions.
You need to choose between proceeding numerically or symbolically. Numerically you would use
and symbolically you would use
Both of those symbolic processes will return two roots for a polynomial of degree 2.
There is no exact equivalent to fzero() for symbolic work.